linux-kernel - [PATCH AUTOSEL 5.15 30/50] printk: Avoid livelock with heavy printk during panic

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Date:   Wed, 30 Mar 2022 07:49:44 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Stephen Brennan <stephen.s.brennan@...cle.com>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Sasha Levin <sashal@...nel.org>, gregkh@...uxfoundation.org
Subject: [PATCH AUTOSEL 5.15 30/50] printk: Avoid livelock with heavy printk during panic

From: Stephen Brennan <stephen.s.brennan@...cle.com>

[ Upstream commit 13fb0f74d7029df3b8137f11ef955e578a4a4a60 ]

During panic(), if another CPU is writing heavily the kernel log (e.g.
via /dev/kmsg), then the panic CPU may livelock writing out its messages
to the console. Note when too many messages are dropped during panic and
suppress further printk, except from the panic CPU. This could result in
some important messages being dropped. However, messages are already
being dropped, so this approach at least prevents a livelock.

Reviewed-by: Petr Mladek <pmladek@...e.com>
Signed-off-by: Stephen Brennan <stephen.s.brennan@...cle.com>
Reviewed-by: Sergey Senozhatsky <senozhatsky@...omium.org>
Signed-off-by: Petr Mladek <pmladek@...e.com>
Link: https://lore.kernel.org/r/20220202171821.179394-4-stephen.s.brennan@oracle.com
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 kernel/printk/printk.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index f9aebe88cc53..f37889b6cf9f 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -93,6 +93,12 @@ EXPORT_SYMBOL_GPL(console_drivers);
  */
 int __read_mostly suppress_printk;
 
+/*
+ * During panic, heavy printk by other CPUs can delay the
+ * panic and risk deadlock on console resources.
+ */
+int __read_mostly suppress_panic_printk;
+
 #ifdef CONFIG_LOCKDEP
 static struct lockdep_map console_lock_dep_map = {
 	.name = "console_lock"
@@ -2222,6 +2228,10 @@ asmlinkage int vprintk_emit(int facility, int level,
 	if (unlikely(suppress_printk))
 		return 0;
 
+	if (unlikely(suppress_panic_printk) &&
+	    atomic_read(&panic_cpu) != raw_smp_processor_id())
+		return 0;
+
 	if (level == LOGLEVEL_SCHED) {
 		level = LOGLEVEL_DEFAULT;
 		in_sched = true;
@@ -2607,6 +2617,7 @@ void console_unlock(void)
 {
 	static char ext_text[CONSOLE_EXT_LOG_MAX];
 	static char text[CONSOLE_LOG_MAX];
+	static int panic_console_dropped;
 	unsigned long flags;
 	bool do_cond_resched, retry;
 	struct printk_info info;
@@ -2661,6 +2672,10 @@ void console_unlock(void)
 		if (console_seq != r.info->seq) {
 			console_dropped += r.info->seq - console_seq;
 			console_seq = r.info->seq;
+			if (panic_in_progress() && panic_console_dropped++ > 10) {
+				suppress_panic_printk = 1;
+				pr_warn_once("Too many dropped messages. Suppress messages on non-panic CPUs to prevent livelock.\n");
+			}
 		}
 
 		if (suppress_message_printing(r.info->level)) {
-- 
2.34.1